105
See also: Henry Cavendish 78–79 ■ Antoine Lavoisier 84 ■ John Dalton 112–13 ■
Jöns Jakob Berzelius 119 ■ Dmitri Mendeleev 174–79T
he Law of Definite
Proportions, published
by French chemist Joseph
Proust in 1794, shows that no
matter how elements combine, the
proportions of each element in a
compound are always precisely the
same. This theory was one of the
fundamental ideas about elements
that emerged at this period to form
the basis of modern chemistry.
In making his discovery, Proust
was following a trend in French
chemistry, pioneered by Antoine
Lavoisier, which advocated careful
measurement of weights, ratios,
and percentages. Proust studied
the percentages in which metals
combined with oxygen in metal
oxides. He concluded that when
metal oxides formed, the proportion
of metal and oxygen was constant.
If the same metal combined with
oxygen in a different proportion, it
formed a different compound with
different properties.
Not everyone agreed with
Proust, but in 1811, the Swedish
chemist Jöns Jakob Berzelius
realized that Proust’s theory fitJohn Dalton’s new atomic theory of
elements—that elements are each
made of their own unique atoms. If
a compound is always made from
the same combination of atoms,
Proust’s argument that elements
always combine in fixed
proportions must be true. This is
now accepted as one of the key
laws of chemistry. ■EXPANDING HORIZONS
ELEMENTS
ALWAYS COMBINE
THE SAME WAY
JOSEPH PROUST (1754–1836)
IN CONTEXT
BRANCH
Chemistry
BEFORE
c.400 BCE The Greek thinker
Democritus proposes that the
world is ultimately made of tiny
indivisible particles—atoms.
1759 English chemist Robert
Dossie argues that substances
combine when they are in the
right proportion, which he calls
the “saturation proportion.”
1787 Antoine Lavoisier and
Claude Louis Berthollet devise
the modern system of naming
chemical compounds.
AFTER
1805 John Dalton shows that
elements are made up of atoms
of a particular mass, which
combine to make compounds.
1811 Italian chemist Amedeo
Avogadro makes a distinction
between atoms and the
molecules that are formed by
atoms to make compounds.
Iron, like many other metals,
is subject to the law of nature
which presides at every
true combination, that
is to say, that it unites
with two constant
proportions of oxygen.
Joseph Proust